- Crystallites can have random or directed orientation.
- Amorphous materials lack structure.
- Polycrystalline materials are composed of multiple crystallites.
- Most inorganic solids are polycrystalline.
- Grain boundaries are the areas where crystallites meet.
- Crystallite size can be determined using X-ray diffraction or electron microscopy.
- Solid objects are rarely composed of a single crystal.
- Crystallite size can vary from nanometers to millimeters.
- Monodisperse microstructures have uniform crystallite size.
- Some materials, like gems and certain fibers, can be single crystals.
Effects on Material Physical Properties
- Crystallinity affects the physical properties of a solid.
- Different allotropic forms of sulfur have different properties.
- Powder grains can be composed of smaller polycrystalline grains.
- Polycrystals cannot be superheated and will melt promptly.
- Material fractures can be intergranular or transgranular.
- Grain boundaries are interfaces where crystals of different orientations meet.
- Grain boundaries contain dislocations and impurities.
- Five variables are required to define a grain boundary.
- Grain boundaries disrupt the motion of dislocations.
- Reducing grain size improves strength without sacrificing toughness.
Grain Boundary Migration
- Grain boundary migration plays a role in creep mechanisms.
- Fine-grained materials have poor resistance to creep at high temperatures.
- Grain boundaries are sources and sinks of point defects.
- Migration rate in grain boundaries depends on the angle between adjacent grains.
- Grain boundaries become a significant volume fraction in nanocrystalline solids.
Crystallite Data Sources